[Mechanism of Hansenula polymorpha reaction to the pulsed addition of a limiting substrate]

As was shown in experiments with a Hansenula polymorpha culture, a temporary drop in the pH of the medium in response to a pulse addition of a limiting substrate (organic or mineral) is not related to NH4+ uptake from the medium. The response is similar in media with NH4+ and in distilled water without NH4+. The pH drop caused by a pulse addition of certain substrates appears to result from the extrusion of H+ ions in the process of antiport: K+/H+ and Mg2+/H+. It is likely that the response to a substrate pulse is the extrusion of H+ ions for maintaining the membrane potential decreased owing to the uniport of either NH4+ or K+. Protons may be extruded in response to a substrate pulse during glycolysis of respiration. It is possible that an addition of organic substrates activates the metabolism; inorganic ions may also have a stimulating action. The lag time from the moment of substrate addition to the beginning of a decrease in the pH of the medium seems to include transport to the cytoplasmic membrane, transport into the cell and, possibly, the first steps of metabolism of the added substrate.